Circular -linear polarizer

Abstract: A circular-linear polarizer comprising a waveguide and a one-quarter wave length plate installed on the inside wall of the waveguide having a close junction between them, the inside wall of the waveguide having a flat part for the flat junction surface of the one-quarter wave length plate, or the junction surface of the one quarter wqve length plate having a circular cross section for a circular inside cross section of the waveguide, improving the electrical contact between them and the cross polarization and axial ratio while keeping the good impedance characteristic.

Description: This invention relates to a circular-linear polarizer used for transmission or reception of the microwave electromagnetic wave, and, more particularly, to a waveguide provided with a one-quarter wave length plate with excellent contacting therebetween and favourable impedance characteristics, cross polarization and axial ratio. Circularly polarized electromagnetic waves which proceed with the electric field vector rotating are widely used in the transmission in the microwave band, as the setting of aerial therefor is easy. In the following, referring to the drawings a circular-linear polarizer of prior art is explained. Fig. 13 and Fig. 14 show a circular-linear polarizer of prior art, the former being a sectional view seen from the direction of the axis of the waveguide (direction of the electromagnetic wave transmission) and the latter being a side section cut by a cutting line S4-S4. As shown, the prior art circular-linear polarizer consists of a waveguide 6 of circular section and a 1/4 wave length phase plate 1 of metal for generating the phase difference of 1/4 wave length. The 1/4 wave length phase plate 1 is, as shown in Fig. 17, trapezoid with a certain thickness, and is mounted with the flat end surface on the inside of the waveguide (in Fig. 13, on the upper side) in the direction of waveguide by screws 5 or such. In such a structure, however, as shown in the partially enlarged section of Fig. 13(b), the phase plate 1 and the circular inside surface 4 of the circular waveguide 6 contact only by the two edges of the end surface of the plate with a gap in between, resulting in a very small contact surface and incomplete grounding, so that favourable input impedance characteristic or cross polarization characteristic were difficult to obtain. Further, a small discrepancy of the position of the phase plate 1 led to a considerable deterioration of the cross polarization characteristics, and difficulty in obtaining a stable characteristics. To constitute a circular-linear polarizer with a phase plate of dielectrics instead of metal is possible. In this case, however, similar difficulties still arose for the exact positioning of the phase plate; the gap and small inexact positioning brought the variation of the characteristics. Thus, in assembly process, adjustment of the mounting position of the phase plate was often necessary. US-A-4,195,270 discloses a dielectric slab polarizer comprising a waveguide whose cross-sectional shape is circular or square and a slab of dielectric material. The surface of the slab is the same shape as the cross-sectional shape of the waveguide where they meet. It is an object of the present invention to obtain a circular-linear polarizer comprising a waveguide and a phase plate having scarcely a gap in between and large contact area, and improved cross polarization characteristics while keeping the favourable impedance characteristics as a waveguide. The circular-linear polarizer according to the present invention, comprises a waveguide and a one-quarter wave length plate, the waveguide having the inside surface whose section consisting of four circular parts and four linear parts arranged alternately, the circular parts being arcs of the same size obtained from one circle and the linear parts also having the same length. The one-quarter wave length plate is preferably a trapezoid of metal of a certain thickness which is installed on a flat part of the inside of the waveguide corresponding to the above described linear part of the section with longer surface abutting the flat junction surface. Further, the circular-linear polarizer of the present invention may comprise a waveguide similar to the above structure and a one-quarter wave length plate of dielectric of H shape with the two vertical lines of H arranged on the two facing flat parts inside the waveguide. Thus the circular linear polarizer of the present invention with the above structure up to the fourth one has no any gap between the junction faces of one-quarter wave length plate and the inside surface of the waveguide but a large junction area, to improve the cross polarization while keeping the input impedance characteristics as a waveguide circuit. The one-quarter wave length plate may have a boss on the joining surface and a waveguide having a hole to correspond to the boss. This keeps the position of the one-quarter wave length plate correct, the polarizer can reduce the deterioration of the cross polarization due to inaccurate positioning, keep stable performance,and save the trouble for adjusting to the exact position or for reassembling. Fig. 1 shows a perspective view of an aerial employing a circular-linear polarizer of the present invention. Fig. 2 shows a fragmentary sectional view of a waveguide circuit constructed with a primary radiator and a circular-linear polarizer of the present invention. Fig. 3 shows a cross section of a circular-linear polarizer of the first embodiment of the present invention, viewed from the axis of the waveguide. Fig. 4 shows a side section of the circular-linear polarizer cut along the S1-S1 line in Fig.3. Fig. 5 shows variation of the axial ratio against input frequency of the circular-linear polarizer of Embodiment 1. Fig. 6 shows the variation of the input impedance of the circular-linear polarizer of Embodiment 1 against the input frequency with the width of the flat part of the waveguide as a parameter. Fig. 7 shows a cross polarization characteristic of an aerial constructed with the circular-linear polarizer of Embodiment 1 as against the angle of rotation of the aerial. Fig. 8 shows a similar cross polarization characteristic of an aerial constructed with the prior art circular-linear polarizer. Fig. 9 shows a cross section of a waveguide same as Embodiment 1 but with a one-quarter wave length plate of a dielectric or a λ/4 dielectric plate viewed from the axis. Fig. 10 shows a longitudinal section of the waveguide shown in Fig. 9 cut along the S2-S2 line therein. Fig. 11 shows a perspective view of a λ/4 metal plate employed in Embodiment 3. Fig. 12 shows a cross-sectional view of a circular-linear polarizer constructed with the metal plate shown in Fig. 11. Fig. 13 shows a cross section of a circular-linear polarizer constructed with a conventional waveguide and a metal plate seen from the axial direction. Fig. 14 shows a section of the circular-linear polarizer shown in Fig. 13 cut along S4-S4 line. Now, referring to the drawings embodiments of the present invention are explained. Referring to Fig. 1, a circular-linear polarizer according to the present invention involved in a converter 10 is applied to a parabola antenna 7 with an arm 9. The converter 10 comprises a waveguide circuit and a converter circuit (not shown), the waveguide consisting of a circular-linear polarizer and a primary radiator. Referring to Fig. 2, showing the inside of the waveguide circuit of converter 10, which comprises a primary radiator 11 with an opening 16, a waveguide 36, a part of which forms a circular-linear polarizer 17, and a exciting probe 12 supported by an insulator 13 on the wall of the waveguide 36. A circularly polarized wave coming into the opening 16 is converted by the circular-linear polarizer 17 to a linearly polarized wave, and transmitted to a converter circuit through the probe 12. Embodiment 1 Referring to Fig. 1, Fig. 2, and Fig. 3(a) showing the cross section of the circular-linear polarizer 17 of the present invention seen from the opening 16, and Fig. 3(b) showing the partial enlargement of the same, the outer surface of the waveguide 36 forms a circular cylinder, whereas the section of the inside surface consists of four circular parts 34 and four linear parts 33 arranged alternately, the lengths of the circular parts being the same, and the lengths of the linear parts being the same. The section is the same along the length of the waveguide, from the opening 16 to the end of the other side. On one of the flat part 33 a one-quarter wave length plate 1 of metal, for example, of aluminum is fixed with two screws 5. The plate 1 is trapezoid with the longer base adjoined to the flat part 33, and the two non-parallel sides starting from the ends of the base being inclined to the opposite direction to avoid the reflection of the incident waves. The plate has a certain thickness and the bottom surface 1a is flat so that no gaps are left between the bottom surface 1a and the flat part 33 of the waveguide inside. The circular-linear polarizer structured as above described synthesizes two linearly polarized elements with circularly polarized waves with 90° different phases by changing with the λ/4 phase plate the length of the wave in the waveguide 36 to produce the phase difference corresponding to a fourth of the wave length. According to the circular-linear polarizer of the embodiment, the flat junction surface 1a of the λ/4 plate is joined to the flat part 33 of the wall of the waveguide, so that no gaps are left in between and large junction area and good earthing are obtained. Embodiment 2 Referring to Fig. 9 and Fig. 10, the cross section of a waqveguide of this Embodiment 2, in which a circular-linear polarizer is formed by employing the wave-shortening effect of dielectric, is the same as that of Embodiment 1. However, different from Embodiment 1, the waveguide 91 is provided with a plate 2 of a dielectric, for example, of fluorocarbon polymers, bridging the opposing two flat parts 93 of the waveguide 91. The plate 2 has a large notch of rectangular form at each side, so that the length the plate 2 in the waveguide axis direction is short in the center and long at the part adjacent to the inner surface of the waveguide, or the plate 2 may be said to have H form with the two side bars of H fixed on the inside of the waveguide. The plate 2 has a certain thickness and is fixed on the flat parts 93 of the wavegide with a binding agent, leaving no gaps. The H shape of the plate 2 contributes to suppress the unfavourable effect due to the reflection of the wave by the plate. The notch, instead of being rectangular, may be triangular, so that the above referred H shape must be understood ---throughout this specification---to mean a rectangular shape with two opposing sides having concave parts. Embodiment 3 Referring to Fig. 11 and 12, the λ/4 metal plate 111 of metal, for example, of aluminum to be installed on the flat inside wall 123 of the waveguide 121 with the screw 5 is provided, on the junction surface, with a boss 15 which is coupled with the hole 122, with or without bottom, provided on the flat surface 123. By such structure, the position of the phase plate is exactly controlled without any variation, and the assembling process is easy and efficient. In Fig.5, the axial ratio by the circular-linear polarizer of Embodiment 1 is shown over the input frequency ranging 11.7 through 12.0 GHz compared to that of the prior art. The axial ratio indicates the ratio of short axis to long axis of the ellipse of the polarized wave; if the ratio is near to 1 or 0 dB it means the ellipse of the polarization is near to a complete circle. The improvement of the axial ratio by the polarizer of Embodiment 1 is observed. In passing the impedance characteristic of this case was favourable as to keep the reflection wave below -23 dB to the incident wave over the frequency range. In Fig. 6, variation of the input impedance of Embodiment 1 various input frequency with the width of the flat part as a parameter; It is observed that the input impedance of the waveguide of the Embodiment 1 and 2 with 3 to 4 mm width of flat part is nearly to same as that of the prior-art circular waveguide and this does not change appreciably over 360° around the waveguide axis showing no any deteriorate effect on the axial ratio by the existence of such flat parts, so that, without λ/4 phase plate, linearly polarized wave can be transmitted or received keeping favourable cross polarization. In passing, the frequency range in Fig. 6 between marks 1 and 2, and between 3 and 4 show BS broadcasting band and CS broadcasting band respectively. Fig. 7 shows a cross polarization of the circular-linear polarizer of Embodiment 1 combined with a parabola antena of 45cm diamter as shown in Fig. 1 rotated around the antena supporting axis over ±90° range at the input frequency 11.85 GHz; the cross polarization on the ordinate is shown as a relative value normalized referring to the level obtained at an optimum condition as to have maximum receiving power for copolarized wave (right-handed circularly polarized wave), whereas Fig. 8 indicates the cross polarization of the prior-art circular-linear polarizer under the same condition as the above present invention of Fig. 7. Comparing the two figures, it is observed that the cross polarization is improved about 4 dB in the vicinity of the main lobe (bore sight) of the aerial radiation pattern. The folded lines in Fig. 7 and Fig. 8 are the CPZ-302 cross polarization curve which is a standard curve defined by Electronics Industrial Asssociation of Japan. Further, the circular-linear polarizer according to the present invention, has an advantage to prevent the deterioration of cross polarization due to inexact installation of the one-quarter wave length plate, and, with no readjusting, productivity is improved. Thus, the circular-linear polarizer according to the present invention, with the flat part with a certain width in the inside wall of the waveguide has no gap between the wall and the phase plate and sufficient contact area can improve cross polarization which is an ability to exclude not-normally polarized wave, while keeping a good input impedance. Furthermore, the present invention by providing a boss and a hole therefor at the junction surface as the Embodiment 3, the performance deterioration due to assembling inexactness is reduced and assembling is easy and no any adjusting is necessary improving the productivity. The waveguide employed in Embodiment 1 and 2 having four flat parts on the inside wall has impedance characteristic and axial ratio not deteriorated, provided the width thereof is appropriate (3 to 4 mm). Therefore, the waveguide of such structure but without one-quarter wave length plate show favourable cross polarization discrimination for the transmission and reception of a linearly polarized wave. Further, the invented waveguide is of a structure as to prevent the rotation of an interposed article, so that it is convenient if a circuit part of the waveguide 2 such as a ferrofeed for receiving linearly polarized waves orthogonal to each other is to be installed. As shown, with the same shape of the junction surface of the phase plate as the inside wall of the waveguide, the gap between them may be eliminated and the earthing may be improved, so that, cross polarization can be improved while keeping the input impedance favourable. Furthermore, as shown in Embodiment 3, with a boss and a hole to receive on the phase plate and the waveguide the deterioration of the performance such as the axial ratio due to inexact assembling may be reduced, stable operation is obtained, assembling is easy and adjustment after assembling is not necessary.

Claims: A circular linear polarizer comprising: (1) a cylindrical waveguide (36) of which inner surface has a cross-sectional shape comprising: (a) four flat portions (33) at least one of which forms a joining surface, wherein the width of each of said four flat portions does not lower the impedance characteristics of said waveguide, and (b) four curved portions (34) interleaved with said four flat portions (33), wherein each curved portion (34) (i) is adjacent to each side of the flat portions out of four flat portions, and (ii) has the joining sides being adjacent in every point without interruption in the longitudinal direction to the sides of said two flat portions. (2) a one-quarter wave length phase plate (1) having a flat portion joined to said joining surface. The circular linear polarizer of claim 1, wherein said phase plate of a one-quarter wave length is shaped into a trapezoid tapered toward the axis center of the waveguide. The circular linear polarizer of claim 1 or 2, wherein a boss (15) is provided on the joining side of the phase plate (1), and a guide recess (122) corresponding to said boss is provided on the joining surface of the waveguide. These circular linear polarizer of claims 1, 2 or 3 wherein said phase plate is conductive.

Inventors: TOKUDA KATSUHIKO, YOSHIMURA YOSHIKAZU, TOKUDA, KATSUHIKO, YOSHIMURA, YOSHIKAZU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1994
Main classification: H01P1
IPC codes: H01P1